What is undetectable viral load?

All viral load tests have a cut-off point below which they cannot reliably detect HIV. This point is called the limit of detection (LOD) and varies from one testing kit to another.

However, just because the level of HIV is too low to be measured by these tests, it does not necessarily mean that the virus has disappeared entirely. The virus may still be present in your blood, but in amounts too low for the test to pick it up. Viral load tests only measure the amount of HIV in the blood. Even if you have an undetectable viral load this does not mean that your viral load in other parts of your body, such as your lymph nodes, is undetectable.

What are the limits of detection of current tests?

For tests used in the past, the lower limit of detection was 400 or 500 copies. However, ultra-sensitive tests are now available that measure down to 50 copies and are routinely used. Some clinics use test that can measure down to 40 copies. Boosted tests can measure much lower levels but these are mainly used in research.

The value of having undetectable viral load

Having undetectable viral load is desirable for two reasons:

• A very low risk of developing AIDS.
• A very low risk of developing resistance to the drugs you are taking now.

Doctors now think that undetectable viral load (below 50 copies) is the aim of treatment.

It may be the case that the quicker your viral load falls below 50 copies, the longer it should stay there, providing you keep taking the drugs as instructed. Some people take three to six months to reach this point, while others go below the limit of detection within four to twelve weeks, and others may never achieve this goal.

After six months on a first-line combination, your viral load should ideally have gone below 50 copies. Some people do not respond this well, however.

If your viral load is not reduced to undetectable levels after three months on a new combination of drugs, some doctors will recommend changing your combination or adding another drug (after they have done a resistance test and found that this drug will work). However, doctors differ in their view of how quickly treatment should be changed and this also depends on the number of HIV treatments you have taken previously. Some favour switching 'early' to reduce the risk of resistance. Others argue that this approach may cause you to stop treatments from which you were still benefiting (see Viral load blips later in this booklet for more information).

People taking anti-HIV drugs for the first time are more likely to reduce their viral load to these very low levels than those who have taken treatment previously.

Viral load blips

People with undetectable viral load are likely to experience small blips in their viral load from time to time. Typically, viral load may rise from below 50 copies to above 100 or 200 copies on a single test, and be undetectable on the next test. This is common and does not necessarily indicate that your treatment is failing. Most viral load blips seem to be due to testing errors at the laboratory.

However, if viral load continues to rise on each test, or if it stays above 50 copies without rising above 500 copies, this indicates that your treatment may fail and you may develop resistance.

At this point you should discuss switching treatment with your doctor. The longer that your viral load remains detectable while you take your current drug combination, the more likely you are to develop resistance to the drugs.

If a second combination seems very likely to reduce your viral load to undetectable levels, then an earlier switch will offer the least possible risk of resistance developing. If you have fewer drug options available, you may be more inclined to switch later. However, if it is possible to switch early with higher CD4 counts and a lower viral load, this is recommended.